Gas turbine engine rotors include a plurality of blades attached to a disk for rotation around the rotational axis of the engine. The disks are attached to a shaft sometimes referred to as "spool". Most modern gas turbine engines include a high-speed shaft and a low speed shaft. The forward end of the low speed shaft is connected to the fan and low-speed compressor and the other end is attached to the low-speed turbine. The forward end of the high-speed shaft is connected to the high-speed compressor and the other end is attached to the high-speed turbine. The shafts are substantially concentric and both are centered on the rotational axis of the engine. Bearings disposed between the shafts and the fixed frame of the engine provide load paths therebetween. The center of gravity of each rotor is designed to coincide with the rotational axis of the engine for load symmetry purposes. Each rotor also has a natural vibratory frequency which, by design, is higher than the rotor's maximum rotational frequency.
To ensure maximum safety, it is common practice to design shafts and accompanying hardware to accommodate possible, but unlikely, shaft loadings. One scenario that must be considered is the partial or complete liberation of a rotor blade. In the event a rotor blade partially or completely liberates from a disk, the rotor will experience an imbalance load that can change the rotor's center of gravity, displacing it from the rotational axis of the engine. Since the bearings constrain the rotor radially, the misalignment between the rotor's center of gravity and the axis of the engine results in the imbalance load being transmitted through the bearings to the support frame. To avoid or minimize support frame damage, it is common practice to make the support frame strong enough to withstand the imbalance load until the engine can be safely shut down. Unfortunately, a support frame strong enough to withstand the largest possible imbalance load is often impractically heavy, particularly with the large fan diameters of today's high bypass gas turbine engines.
An alternative approach to accommodating a rotor imbalance load is to support a bearing with a bolted flange arrangement. If the imbalance load exceeds a predetermined limit, the bolts shear thereby preventing the imbalance load from traveling through the bearing to the support frame. A problem with this approach is that the failed bolts can be liberated and sent aft into the engine where they themselves can cause foreign object damage.
Another approach for accommodating a rotor imbalance load is to use a bearing support designed to buckle when subjected to an imbalance load greater than a predetermined limit. One potential drawback of this approach is the difficulty of analytically predicting when the support will buckle given the variety of load scenarios that may occur. The analytical predictions often must be substantiated through extensive, expensive empirical testing. Another potential drawback is that "buckling" bearing supports typically require tight manufacturing tolerances and strict material properties, both of which add to the cost of the support.
What is needed, therefore, is an apparatus and/or a method for supporting a rotatable shaft within a gas turbine engine that can accommodate an imbalance load, one that does not create foreign object damage, one that is readily manufacturable, and one that is cost-effective.